<select name="{$field.name}" class="form-control" style="width:auto;">
    <option></option>
    <volist name="field.opt" id="option">
        <php>
            $selected = $field['value']==$key ? 'selected' : '';
        </php>
        <option value="{$key}"
                {$selected}>{$option|htmlspecialchars}
        </option>
    </volist>
</select>